# C++ named requirements: LegacyForwardIterator

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A LegacyForwardIterator is a LegacyIterator that can read data from the pointed-to element.

Unlike LegacyInputIterator and LegacyOutputIterator, it can be used in multipass algorithms.

If a LegacyForwardIterator it originates from a Container, then it's value type is the same as the container's, so dereferencing (*it) obtains the container's value type.

### Requirements

Type | Definition
`X` | A forward iterator type
`T` | The value type of `X` (i.e. std::iterator_traits<X>::value_type)
`Ref` | std::iterator_traits<X>::reference
Value | Definition
i, j | Values of type `X` or const X
r | A value of type `X&`

`X` satisfies LegacyForwardIterator if all following conditions are satisfied:

- `X` satisfies LegacyInputIterator.
- `X` satisfies DefaultConstructible.
- If `X` is a mutable iterator, `Ref` is a reference to `T`.
- If `X` is a constant iterator, `Ref` is a reference to const T.
- Objects of the type `X` provide multi-pass guarantee.
- If i and j are equal, then either i and j are both dereferenceable or else neither is dereferenceable.
- If i and j are both dereferenceable, then i == j if and only if *i and *j are bound to the same object.
- The following expressions must be valid and have their specified effects:

Expression | Type | Effects
--- | --- | ---
r++ | convertible to const X& | Equivalent to X x = r; ++r; return x;.
*i++ | `Ref` |

### Equality domain

The domain of == for forward iterators is that of iterators over the same underlying sequence.

However, value-initialized forward iterators can be compared, and must compare equal to other value-initialized iterators of the same type. In other words, value-initialized forward iterators behave as if they refer past the end of the same empty sequence. | (since C++14)

### Multi-pass guarantee

Two dereferenceable iterators a and b of type `X` offer the multi-pass guarantee if all following conditions are satisfied:

- a == b implies ++a == ++b.
- Any of the following conditions is satisfied:

  - `X` is a pointer type.
  - The expression (void)++X(a), *a is equivalent to the expression *a.

### Concept For the definition of std::iterator_traits, the following exposition-only concept is defined.
template<class It> concept __LegacyForwardIterator = __LegacyInputIterator<It> && std::constructible_from<It> && std::is_reference_v<std::iter_reference_t<It>> && std::same_as< std::remove_cvref_t<std::iter_reference_t<It>>, typename std::indirectly_readable_traits<It>::value_type> && requires(It it) { { it++ } -> std::convertible_to<const It&>; { *it++ } -> std::same_as<std::iter_reference_t<It>>; }; |  |
where the exposition-only concept `__LegacyInputIterator<T>` is described in LegacyInputIterator. | (since C++20)

### Notes

Unlike the std::forward_iterator concept, the LegacyForwardIterator requirements requires dereference to return a reference.

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 1212 (N3066) | C++98 | the type of *i++ did not match the type of *i-- required by LegacyBidirectionalIterator | changed the type to `Ref`
LWG 1311 (N3066) | C++98 | “a == b implies ++a == ++b” alone did not offer multipass guarantee[1] | also requires “a == b implies ++a != b”[2]
LWG 3798 | C++20 | `__LegacyForwardIterator` required std::iter_reference_t<It> to be an lvalue reference type | also allows rvalue reference types

1. ↑ In the scenario where a and b use the same underlying iterator, evaluating the expression ++a == ++b actually increments the underlying container twice, but the result is still true.
2. ↑ Formally also requires implying ++b != a.

### See also

forward_iterator (C++20) | specifies that an `input_iterator` is a forward iterator, supporting equality comparison and multi-pass (concept)
Iterator library | provides definitions for iterators, iterator traits, adaptors, and utility functions

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